1. Field of the Invention
The invention concerns an intervertebral disc prosthesis.
2. Description of the Related Art
The intervertebral discs serve as elastic support upon compression between the vertebrae elements of the spinal column. Damage of the intervertebral discs, in particular resulting from degeneration and wear, may lead to severe limitations of mobility and neurological symptoms, in particular pain and paralysis. If such diseases cannot be cured conservatively any more, it is known to fuse the vertebrae elements with each other. Admittedly, this results in the affected vertebrae not being movable relative to each other any longer, so that this leads to a stiffening of the spinal column.
As an alternative, the damaged intervertebral disc may be replaced by an intervertebral disc prosthesis according to the invention. Such intervertebral disc prosthesis consists of an upper cranial disc and a lower caudal disc, between which spring means are inserted, which support these discs elastically upon compression relative to each other. The intervertebral disc prosthesis is inserted between the vertebrae in place of the removed intervertebral disc, whereas the upper and lower discs are anchored to the vertebrae elements of the superior and inferior vertebra. In an intervertebral disc prosthesis, which is known from EP 0 706 354 B1, the spring means are formed by an elastic synthetic cushion. In an intervertebral disc prosthesis, which is known from FR 2 734 148 A1, the spring means are formed by a spring made of titanium or steel, which characteristic response curve causes an exponential increase of the spring tension with decreasing vertebrae distance. These known intervertebral disc prosthesis permit axial and torsion mobility of the vertebrae. Disadvantageously, the material of the spring means is liable to fatigue, which leads to a reduction in of effectiveness of the intervertebral disc prosthesis, and with metal springs, yet may lead to breakage.
It is the task of the invention to provide an intervertebral disc prosthesis, which combines good mobility of the vertebrae with long-lasting effectiveness.
The substantial idea of the invention is to manufacture the spring means of the intervertebral disc prosthesis using a memory-metal alloy, which comprises super-elastic properties at body temperature. The super-elasticity, which is also named pseudo-elasticity, is based on a tension-induced conversion of austenite to martensite. This reversible conversion of the crystal structure allows a substantially greater elastic deformation in contrast to the Hooke""s elasticity of conventional materials and in particular conventional metals, like e.g. steel or titanium. Since the super-elastic deformation is based on a conversion of the crystal structure and not, as with the conventional Hooke""elasticity, on a deformation of the crystal structure, the super-elastic deformation does not lead to fatigue of material. Thus, the elastic properties of the intervertebral disc prosthesis remain unchanged, even after long periods of implantation and a corresponding high number of load alterations, and fatigue break of the spring means need not be feared.
The use of memory-metal alloy for the manufacture of the spring means allows great latitude in the design of the spring means. These may be fine-tuned with respect to their elastic properties and their spring tension. Optimal adjustment of the spring characteristic response curve with respect to both axial movement and torsion movement of the vertebrae is possible.
In one embodiment, the spring means may be in the form of a helical compression spring. Such compression spring allows a particular favorable mobility of the vertebrae with respect to axial movements, tilt movements and torsion movements.
If the spring means is in the form of a disk spring, a good axial and tilt mobility may be realized, together with increased stiffness, where appropriate, whereas no or only slight torsion mobility exists.
In a further embodiment, the spring means may be formed by a spiral spring, which lies between the discs in the form of a toroid or annular spring. Such spiral spring allows axial mobility by means of pressing the spiral spring vertically along its axis. Tilting the coils of the spiral spring towards its axis allows torsion movement. Finally, the spring means may also be formed by one or more flat springs. With such flat springs, particular soft axial shock absorption is obtained. Torsion movements are more or less possible, depending on the width of the flat springs.
The intervertebral disc prosthesis is preferably inserted for that region of the spinal column, which requires a particular high mobility, i.e. in particular for the region of the lumbar vertebrae and the lower thoracic vertebrae. The construction and the dimension of the spring means may therein be chosen according to the application of the intervertebral disc prosthesis, in particular according to the required axial elasticity and the desired torsion mobility.
Preferably, a flexible protective coating covers the spring means formed by the memory-metal alloy in order to prevent tissue from growing into it, which could impair the spring characteristics of the spring means. The protective coating preferably consists of a biocompatible continuously elastic synthetic material.